A secondary battery attracts attention as a power source of an electric vehicle (EV), a hybrid electric vehicle (HEV), a parallel hybrid electric vehicle (PHEV), and the like, which have been suggested as alternatives for solving defects such as environmental contamination due to commonly used gasoline vehicles, diesel vehicles, and the like using fossil fuels. In a medium and large size device such as automobiles, a medium and large size battery module in which a plurality of battery cells is electrically connected is used due to the need of high power and high capacity.
However, since the medium and large size battery module is necessary to be manufactured so as to have a small size and a light weight, a square shape battery, a pouch shape battery, etc., which may be stacked in a high degree and have a light weight when compared with the capacity, are widely used as the battery cells of the medium and large size battery module.
Generally, an electrode assembly may be classified according to the structure of the electrode assembly having cathode/separator/anode. Typically, the electrode assembly may be classified into a jelly-roll (a wrapping type) electrode assembly, in which cathodes and anodes having long sheet shapes along with an interposed separator are wrapped, a stack type (a laminated type) electrode assembly, in which a plurality of cathodes and anodes along with interposed separators, which are cut into specific size units and stacked one by one, and a stack/folding type electrode assembly. The stack/folding type and the stack type electrode assemblies are typically used, and defects on each structure will be explained.
First, the stack/folding type electrode assembly disclosed in Korean Patent Application Publication Nos. 2001-0082058, 2001-0082059 and 2001-0082060 filed by the present Applicant will be explained.
Referring to FIG. 13, an electrode assembly 1 of a stack/folding structure includes a plurality of overlapped full cells 2, 3, 4 . . . (Hereinafter, will be referred to as ‘full cell’) as a unit cells, in which cathode/separator/anode are positioned in sequence. In each of the overlapped parts, a separator sheet 5 is interposed. The separator sheet 5 has a unit length possibly wrapping the full cells. The separator sheet 5 initiated from the central full cell 10 is bent inward by the unit length while continuously wrapping each of the full cells to the outermost full cell 14 so as to be interposed in the overlapped parts of the full cells. The distal end portion of the separator sheet 5 is finished by conducting heat welding or attaching using an adhesion tape 6. The stack/folding type electrode assembly is manufactured by, for example, arranging the full cells 2, 3, 4 . . . on the separator sheet 5 having a long length and wrapping from one end portion of the separator sheet 5 in sequence. However, in this structure, a temperature gradient may be generated between the electrode assemblies 1a, 1b and 2 in the center portion and the electrode assemblies 3 and 4 disposed at the outer portion to produce different heat emitting efficiency. Thus, the lifetime of the electrode assembly may be decreased when used for a long time.
The manufacturing process of the electrode assembly is conducted by using two lamination apparatuses for manufacturing each electrode assembly and one additional folding apparatus as a separate apparatus. Therefore, the decrease of the tack time of the manufacturing process has a limitation. Particularly, the minute aligning of the electrode assemblies disposed up and down is difficult in the structure accomplishing the stacked structure through the folding. Thus, the manufacture of an assembly having a reliable quality is very difficult.
FIG. 14 illustrates the structures of A type and C type bicells different from the full cell structure as a unit cell applicable in the above-described folding structure in FIG. 13. At the center portion which is an initiating point of wrapping among the overlapped electrochemical cells applicable in the present invention, a bicell having a cathode/separator/anode/separator/cathode structure (a) (A type bicell), or a bicell having a anode/separator/cathode/separator/anode structure (b) (C type bicell), surrounded by a separator sheet is disposed. That is, the structure of a common bicell may be accomplished by ‘A type bicell’ having a stacked structure of a double sided cathode 10, a separator 20, a double sided anode 30, a separator 40, a double sided cathode 50 one by one as illustrated in FIG. 14 (a), or a stacked structure of a double sided anode 30, a separator 20, a double sided cathode 10, a separator 40, and a double sided anode 50 one by one as illustrated in FIG. 14 (b).
For the structure of the electrode assembly using the folding process, a folding apparatus is separately necessary. When a bicell structure is applied, two types of the bicells of the A type and the C type, are manufactured and stacked. Before conducting the folding, the keeping of the distance between one bicell and another bicell disposed on a long separator sheet is a very difficult task. That is, an accurate alignment between the upper and lower unit cells may be difficult. When manufacturing a high capacity cell, a considerable time may be necessary for changing the types.
Next, a stack type electrode assembly will be explained. Since the stack type electrode assembly is widely known in this art, only on the defects of the stack type electrode assembly will be explained in brief.
Generally, in the stack type electrode assembly, the length and the width of a separator are greater than those of an electrode. The separator is stacked on a magazine or a jig having corresponding size with respect to the length and the width of the separator, and the stacking process of the electrode on the separator is repeatedly conducted to manufacture the stack type electrode assembly.
However, when the stack type electrode assembly is manufactured by the above-described method, the electrode and the separator are necessary to be stacked one by one. Thus, the working time is increased to remarkably lower the productivity. In addition, the alignment of the plurality of the separators by the length and the width is possible. However, since the magazine or the jig accurately aligning the electrodes put on the separator is not present, the plurality of the electrodes provided in the stack type electrode assembly may not be aligned but may be dislocated.
In addition, since the faces of the cathode and the anode across the separator are dislocated, an electrochemical reaction may not be made in a portion of the active material region coated on the surfaces of the cathode and the anode. Thus, the efficiency of a battery cell may be deteriorated.